The present invention relates to compatibilized blends comprising high and low unsaturation elastomers.
Vulcanizates based on blends of elastomers which contain little or no olefinic unsaturation with more highly unsaturated elastomers are of interest in the rubber industry primarily because of their special properties, e.g., superior resistance to ozone degradation and consequent cracking, improved resistance to chemical attack, improved temperature stability and unique dynamic response. These blends can permit the achieving of synergisms wherein the composite blend possesses combinations of properties unattainable in the individual elastomers. However, these desirable properties can be realized only when an intimate homogeneous blend of the elastomers with phase sizes of less than 5 microns, generally 1 micron or less, is produced and maintained in the blend and a satisfactory level of interfacial adhesion is achieved.
Unfortunately, it is generally known that most polymers are not compatible with one another unless specific favorable interactions are present because the favorable entropy of mixing is too small to overcome the unfavorable enthalpy of mixing, thus making the free energy of mixing unfavorable. Blends produced by normal techniques are grossly inhomogeneous with phase sizes many microns in diameter. This gross incompatibility of the individual polymers with a consequent inability to produce and maintain the homogeneous fine phase sizes required in synergistic blends is particularly evidenced when the individual polymers differ considerably in solubility parameters as is the case when attempts are made to blend low unsaturation elastomers with the more highly unsaturated elastomers.
A further problem with mixtures of saturated and unsaturated elastomers is that, even if intimate dispersions can be produced during high shear mixing operations, the mixtures phase-separate when the mixing is stopped so that the final blends are grossly inhomogeneous with the individual phase sizes many microns in diameter. These grossly inhomogeneous blends generally have very poor combinations of properties, usually much worse than the individual polymers, rather than displaying the desirable synergistic combination of properties obtainable in the more intimate homogeneous blends of phase sizes less than 5 microns, preferably 1 micron or less.
One approach towards compatibilizations of non-compatible polymers is to include in the blend a block copolymer which contains one chain segment derived from monomers compatible with one blend polymer and another chain segment derived from monomers compatible with the other blend polymer. For example, EP691378A discloses polymer blends comprising a polycarbonate resin and polyisobutylene which are compatibilized by including a minor amount of a polycarbonate-polyisobutylene block copolymer in the blend composition. In addition, U.S. Pat. No. 5,741,859 discloses block copolymers of polyisobutylene and polydimethylsiloxane and suggests their use as compatibilizers.
Di, tri and radial block copolymers containing polyisobutylene and poly(p-chlorostyrene) are disclosed as compatibilizers by Kennedy et al. in Polym.Mater.Sci.Eng., 63, p 371-375, 1990.
Polymer blends comprising a mixture of an isobutylene polymer and a more highly unsaturated elastomer such as polybutadiene or polyisoprene are extremely interesting because of the potential improvement of such properties as ozone resistance, resistance to chemical attack, resistance to air permeability, and improved temperature stability as described above. But isobutylene polymers such as polyisobutylene, copolymers of isobutylene with isoprene (butyl rubber), copolymers of isobutylene with a para-alkylstyrene and halogenated versions thereof are not very compatible with more highly unsaturated elastomers such as polymers based on conjugated diene monomers. In the absence of specific, strong chemical interactions, two dissimilar polymers of this type have a positive free energy of mixing and hence are thermodynamically incompatible because the heat of mixing is usually positive and the entropy gained upon mixing these dissimilar polymeric molecules is quite small. The result is a high interfacial tension and poor adhesion between the two phases in the blend, and weak blend mechanical properties due to lack of a highly structured morphology. The present invention is directed towards an improved blend of high and low unsaturation elastomers by the incorporation of a block copolymer.
The invention provides a compatibilized polymer blend comprising:
(a) an isobutylene polymer selected from the group consisting of polyisobutylene, random copolymers of isobutylene with up to about 10 weight % isoprene, halogenated random copolymers of isobutylene with up to about 10 weight % isoprene, random copolymers of isobutylene with up to 20 weight % of a para-alkylstyrene, halogenated random copolymers of isobutylene and up to 20 weight % of a para-alkylstyrene and mixtures thereof;
(b) at least one olefinically unsaturated diene polymer; and
(c) a compatibilizer for components (a) and (b) comprising a block copolymer of at least one recurring polyisobutylene segment and at least one recurring segment comprising a C4 to C6 alkyl ring substituted styrene or ring-substituted alpha-methylstyrene.
The preferred compatibilizer comprises a di, tri or radial block copolymer of polyisobutylene and para-tertiary-butylstyrene (tbS).
The invention also provides for co-vulcanized elastomer compositions based on these blends.